1. while loops; file I/O; introduction to lists
Week 6
while loops; file I/O; introduction to lists
Special thanks to Scott Shawcroft, Ryan Tucker, and Paul Beck for their work on these slides.
Except where otherwise noted, this work is licensed under:

2. while Loops while test: statements sentinel.py 1 2 3 4 5 6 7 8 9 10
# Sums integers entered by the user
# until -1 is entered, using a sentinel loop.
sum = 0
number = input("Enter a number (-1 to quit)? ")?
while number != -1:
sum += number
print "The total is", sum

3. Random Numbers from random import * randint(min, max) choice(sequence)
returns a random integer in range [min, max] inclusive
choice(sequence)
returns a randomly chosen value from the given sequence
(the sequence can be a range, a string, an array, ...)
>>> from random import *
>>> randint(1, 5) 2 5
>>> choice(range(4, 20, 2)) 16
>>> choice("hello")
'e'

4. Reading Files name = open("filename")
opens the given file for reading, and returns a file object
name.read() - file's entire contents as a string
name.readline() - next line from file as a string
name.readlines() - file's contents as a list of lines
the lines from a file object can also be read using a for loop
>>> f = open("hours.txt")
>>> f.read()
'123 Susan \n
456 Brad \n
789 Jenn \n'

5. File Input Template A template for reading files in Python:
name = open("filename")
for line in name:
statements
>>> input = open("hours.txt")
>>> for line in input:
print line.strip() # strip() removes \n
123 Susan
456 Brad
789 Jenn

6. Exercise
Write a function input_stats that accepts a file name as a parameter and that reports the longest line in the file.
example input file, carroll.txt:
Beware the Jabberwock, my son,
the jaws that bite, the claws that catch,
Beware the JubJub bird and shun
the frumious bandersnatch.
expected output:
>>> input_stats("carroll.txt")
longest line = 42 characters
the jaws that bite, the claws that catch,

7. Exercise Solution def input_stats(filename): input = open(filename)
longest = ""
for line in input:
if len(line) > len(longest):
longest = line
print "Longest line =", len(longest)
print longest

8. Lists list: Python's equivalent to Java's array (but cooler)
Declaring:
name = [value, value, ..., value] or,
name = [value] * length
Accessing/modifying elements: (same as Java)
name[index] = value
>>> scores = [9, 14, 18, 19, 16]
[9, 14, 18, 19, 16]
>>> counts = [0] * 4
[0, 0, 0, 0]
>>> scores[0] + scores[4] 25

9. Indexing Lists can be indexed using positive or negative numbers:
>>> scores = [9, 14, 12, 19, 16, 18, 24, 15]
[9, 14, 12, 19, 16, 18, 24, 15]
>>> scores[3] 19
>>> scores[-3] 18
index 1 2 3 4 5 6 7
value 9 14 12 19 16 18 24 15 -8 -7 -6 -5 -4 -3 -2 -1

10. Slicing slice: A sub-list created by specifying start/end indexes
name[start:end] # end is exclusive
name[start:] # to end of list
name[:end] # from start of list
name[start:end:step] # every step'th value
>>> scores = [9, 14, 12, 19, 16, 18, 24, 15]
>>> scores[2:5]
[12, 19, 16]
>>> scores[3:]
[19, 16, 18, 24, 15]
>>> scores[:3]
[9, 14, 12]
>>> scores[-3:]
[18, 24, 15]
index 1 2 3 4 5 6 7
value 9 14 12 19 16 18 24 15 -8 -7 -6 -5 -4 -3 -2 -1

11. Other List Abilities
Lists can be printed (or converted to string with str()).
Find out a list's length by passing it to the len function.
Loop over the elements of a list using a for ... in loop.
>>> scores = [9, 14, 18, 19]
>>> print "My scores are", scores
My scores are [9, 14, 18, 19]
>>> len(scores) 4
>>> total = 0
>>> for score in scores:
print "next score:", score
total += score
assignment score: 9
assignment score: 14
assignment score: 18
assignment score: 19
>>> total 60

12. Ranges, Strings, and Lists
The range function returns a list.
Strings behave like lists of characters:
len
indexing and slicing
for ... in loops
>>> nums = range(5)
>>> nums
[0, 1, 2, 3, 4]
>>> nums[-2:]
[3, 4]
>>> len(nums) 5

13. String Splitting split breaks a string into a list of tokens.
name.split() # break by whitespace
name.split(delimiter) # break by delimiter
join performs the opposite of a split
delimiter.join(list)
>>> name = "Brave Sir Robin"
>>> name[-5:]
'Robin'
>>> tokens = name.split()
['Brave', 'Sir', 'Robin']
>>> name.split("r")
['B', 'ave Si', ' Robin']
>>> "||".join(tokens)
'Brave||Sir||Robin'

14. Tokenizing File Input
Use split to tokenize line contents when reading files.
You may want to type-cast tokens: type(value)
>>> f = open("example.txt")
>>> line = f.readline()
>>> line
'hello world \n'
>>> tokens = line.split()
>>> tokens
['hello', 'world', '42', '3.14']
>>> word = tokens[0]
'hello'
>>> answer = int(tokens[2]) 42
>>> pi = float(tokens[3])
3.14

15. Exercise Recall the hours.txt data:
123 Susan
456 Brad
789 Jenn
Recreate the Hours program from lecture in Python:
Susan (ID#123) worked 31.4 hours (7.85 / day)
Brad (ID#456) worked 36.8 hours (7.36 / day)
Jenn (ID#789) worked 39.5 hours (7.9 / day)

16. Exercise Answer hours.py 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
file = open("hours.txt")
for line in file:
tokens = line.split()
id = tokens[0]
name = tokens[1]
# cumulative sum of this employee's hours
hours = 0.0
days = 0
for token in tokens[2:]:
hours += float(token)
days += 1
print name, "(ID#" + str(id) + ") worked", \
hours, "hours (" + str(hours / days), "/ day)"

17. Exercise Recall the Internet Movie Database (IMDb) data:
The Shawshank Redemption (1994)
The Godfather: Part II (1974)
Casablanca (1942)
Recreate the Movies program from lecture in Python:
Search word? part
Rank Votes Rating Title
The Godfather: Part II (1974)
The Departed (2006)
The Apartment (1960)
Spartacus (1960)
4 matches.

18. Exercise Answer movies.py 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
search_word = raw_input("Search word? ")
matches = 0
file = open("imdb.txt")
for line in file:
tokens = line.split()
rank = int(tokens[0])
rating = float(tokens[1])
votes = int(tokens[2])
title = " ".join(tokens[3:])
# does title contain searchWord?
if search_word.lower() in title.lower():
matches += 1
print rank, "\t", votes, "\t", rating, "\t", title
print matches, "matches."

19. Writing Files name = open("filename", "w")
name = open("filename", "a")
opens file for write (deletes any previous contents) , or
opens file for append (new data is placed after previous data)
name.write(str) - writes the given string to the file
name.close() - closes file once writing is done
>>> out = open("output.txt", "w")
>>> out.write("Hello, world!\n")
>>> out.write("How are you?")
>>> out.close()
>>> open("output.txt").read()
'Hello, world!\nHow are you?'